A secondary battery is a battery that is able to convert chemical energy into electrical energy and discharge the energy. Moreover, it is also a battery that is able to convert electrical energy into chemical energy and store (charge) the chemical energy, by passing electrical current in a direction that is opposite to the direction of current at the time of discharge. Of secondary batteries, secondary batteries as typified by lithium secondary battery have high energy density, so that they are widely used as a power source for portable devices such as laptop personal computer, cellular phone, etc.
In a lithium secondary battery, when graphite (C) is used as an anode active material, a reaction represented by the following formula (I) proceeds at the anode, upon discharge:LixC6→6C+xLi++xe−  (I)wherein 0<x<1.
Electrons generated by the reaction of the formula (I) pass through an external circuit, work by an external load, and then reach the cathode. Lithium ions (Li+) generated by the reaction of the formula (I) are transferred by electro-osmosis from the anode side to the cathode side, through an electrolyte sandwiched between the anode and the cathode.
When lithium cobaltate (Li1-xCoO2) is used as a cathode active material, a reaction represented by the following formula (II) proceeds at the cathode, upon discharge:Li1-xCoO2+xLi++xe−→LiCoO2  (I)wherein 0<x<1.
Upon charging the battery, reactions which are reverse to the reactions represented by the above formulae (I) and (II) proceed at the anode and the cathode. At the anode, graphite in which lithium has been intercalated by graphite intercalation (LixC6) becomes reusable, while lithium cobaltate (Li1-xCoO2) is regenerated at the cathode. Because of this, discharge becomes possible again.
Electrode active materials used in lithium batteries are materials that are important for determining the charge-discharge performance of the batteries, and various studies have been conducted. For example, an active material is disclosed in Patent Literature 1, comprising particles which contain a compound having a composition represented by LiaMXO4, which have a given average primary particle diameter, and which are carried by carbon particles.